The very clear waters of the South Pacific Gyre likely constitute an end-member of oligotrophic conditions which remain essentially unknown with respect to its impact on carbon fixation and exportation. We describe a non-intrusive bio-optical method to quantify the various terms of a production budget (Gross Community Production, community losses, net community production) in this area. This method is based on the analysis of the diel cycle in Particulate Organic Carbon (POC), derived from high frequency measurements of the particle attenuation coefficient <i>c<sub>p</sub></i>. We report very high integrated rates of Gross Community Production within the euphotic layer (average of 846&plusmn;484 mg C m<sup>&minus;2</sup> d<sup>&minus;1</sup> for 17 stations) that are far above any rates determined using incubation techniques for such areas. Furthermore we show that the daily production of POC is essentially balanced by the losses so that the system cannot be considered as net heterotoph. Our results thus agree well with geochemical methods, but not with incubation studies based on oxygen methods. We stress to the important role of deep layers, below the euphotic layer, in contributing to carbon fixation when incident irradiance at the ocean surface is high (absence of cloud coverage). These deep layers, not considered up to now, might fuel a part of the heterotrophic processes in the upper layer, in particular through dissolved organic carbon release. We further demonstrate that, in these extremely clear and stratified waters, integrated Gross Community Production is proportional to the POC content and surface irradiance via an efficiency index &psi;<sub>GCP</sub><sup>*</sup>, the water column cross section for Gross Community Production. We finally discuss our results in the context of the role of oligotrophic gyre in global carbon budget and of the possibility of using optical proxy from space for the development of gross community rather than primary production global models.

3095 5 The average GCP within the euphotic zone over four diel cycles is ∼734 (±97) mg C m−2 d−1 (Table 1). This production is balanced by losses [721 (±159) mg C m−2 d−1], and average NCP (over 4 days) is not significantly different from zero.

On days 1 and 3, using classical measurements of O2 changes in light/dark bottles incubated over a 24-h period, Gross Primary Production (O2-GPP) and Community 10 Respiration (O2-CR) were determined in the euphotic zone. O2-GPP, once converted to carbon units with a photosynthetic quotient of 1.1 (Laws, 1991), is significantly lower [473 (±223) mg C m−2 d−1] than GCP (Table 1). By contrast, and assuming a respiratory quotient of 1.1, the CR of 768 (±65) mg C m−2 d−1 is in the same range as the loss rates determined by the optical method.

15 The net community production computed solely using O2 measurements leads to the usual conclusion that the upper ocean is in metabolic deficit (O2-NCP = -295 (±158) mg C m−2 d−1) (Table 1). Being the most oligotrophic oceanic region, it is not surprising that our O2 based production rates are lower than those previously reported for the less oligotrophic North Pacific Gyre [O2-GPP = 657 (±52) mg C m−2 d−1; O2-CR = 20 920 (±39) mg C m−2 d−1]. Our O2-NCP values for the SPG are however essentially equivalent to those for the North Pacific Gyre [-270 (±49) mg C m−2 d−1]. Contrary to incubation methods, our optical method is non-intrusive as are geochemical approaches. In addition, unlike geochemical observations, we integrate over the same time scales (hours to day) as incubation measurements.

Gardner, W. D., Walsh, I. D., and Richardson, M. J.: Biophysical forcing of particle production and distribution during a spring bloom in the North Atlantic. Deep Sea Research Part II: Topical Studies in Oceanography, 40, 171-195, 1993.